How NPL time works

UTC(NPL) and the National Physical Laboratory

The National Physical Laboratory (NPL) in Teddington is the UK's national measurement institute. It keeps UTC(NPL), the UK's realisation of Coordinated Universal Time (UTC), the world's reference time scale. UK civil time is UTC in winter (GMT) and UTC + 1 hour in summer (BST).

UTC itself is calculated internationally by combining atomic clocks from laboratories around the world, including NPL's. NPL's time scale is kept very closely aligned with it.

Atomic clocks, briefly

Since 1967 the second has been defined by a property of the caesium atom: the frequency of radiation it absorbs when switching between two energy states. Atomic clocks count that frequency, which is why they are vastly more stable than quartz or mechanical watches. Laboratories like NPL use them to keep their national time scales accurate.

How this site gets the time

1. Our server asks NPL's public time servers (ntp1.npl.co.uk, then ntp2.npl.co.uk) for the time using the standard Network Time Protocol (SNTP). It measures the round trip and corrects for it.
2. Your browser asks our server for that corrected time and measures its own round trip. It assumes the reply took about half that round trip to arrive, and adds it on.
3. Your browser then runs the clock locally, using its precise internal timer, and checks with the server again every ten minutes (sooner after a failure).

Your browser never talks to an atomic clock or to NPL directly. It receives a reference time that our server obtained from NPL's public service a moment earlier.

Network delay, uncertainty and your device's clock

• Estimated network uncertainty(shown on the home page as ± ms) comes from the round trips. Because we can't know exactly how long each direction took, the true time could be a little either side of what's shown.
• Device-clock offsetis something different: how far your phone or computer's own clock is from the synchronised time. Devices sync occasionally and drift in between, and some are set manually, so being a few hundred milliseconds or more out is common.
• Your screen's refresh rate, browser and system load add a little more delay to what you see.

Here's the live status for this device right now:

What we can and can't guarantee

• When the status says Synced to NPL, the time came from NPL's public service within the last few minutes, with the uncertainty shown.
• If the time service can't be reached, the site says Using device time and shows your device's own clock, which may be off. It retries automatically.
• Offline, there is no synchronisation at all: any time shown comes from your device.
• This is a helpful setting reference, not a certified or legally traceable time source.

More from NPL: NPL Time & Frequency. Ready to use it? Set your watch.